Find Palindrome Sub-string

Problem Overview

• Find a sub-string that reads the same forwards and backwards (palindrome) in a given string.
• Input: one uppercase A–Z string; Output: a palindromic sub-string of length > 1, or None.
• Selection rules: choose maximum length; if tied, pick the lexicographically smallest.
• Domain: string processing/puzzle inspired by Ray’s challenge.
• Asked in Cisco interviews; a classic coding interview problem and interview question.